gitignore: move "barebox_default_env*" to subdirectory ...

"barebox_default_env*" is not "Top-level generic files" because they
are generated in the defaultenv/ directory since commit 7fa10256c3f4
(defaultenv: Allow multiple defaultenvironment overlays).

The comment block of the top-level .gitignore says as follows:

  # NOTE! Don't add files that are generated in specific
  # subdirectories here. Add them in the ".gitignore" file
  # in that subdirectory instead.

Let's follow this rule.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>